[Qemu-ppc] enhancements to the open firmware video driver

[Programmingkid]

I thought of a way for the video driver you made to be expandable without 
having to be recompiled. The user can specify additions resolutions like this:

-prom-env "resolutions=1200x700,1280x600,1440x700"

The video driver can then look in the /options node and use its resolutions 
property to retrieve the additional resolutions. These resolutions can then be 
made available to the guest.